Laser illumination shows some promise for improving color gamut and achieving needed levels of brightness for digital projection apparatus, including digital projectors capable of providing cinema-quality imaging and pico-projectors offering portable projection for easier sharing of images. One recognized problem with projection systems using narrow band light sources, however, relates to speckle.
Speckle is a fine scale spatially varying intensity fluctuation that is caused by random roughness of optical surfaces on the order of a wavelength of light. The increased coherence of lasers introduces a significant effect in projection systems where the roughness creates randomly phased sub-sources, which interfere together. This random intensity fluctuation lowers the effective quality of an image, especially at the higher frequencies essentially producing a “shimmer effect” that masks fine details, but also creating an intensity sharpness that is really artificial.
The phenomenon of speckle has been studied in detail by many researchers and a comprehensive summary of knowledge has been published by Joseph Goodman in the book “Speckle Phenomena in Optics, Theory and Application” (Roberts and Company Publishers, Greenwood Village, Colo., 2007). Goodman suggests that full-frame displays should have speckle levels where the standard deviation of the intensity variation is less than the magnitude of the least significant bit of the intensity resolution of the modulation device. For Digital Cinema applications 12 bit intensity resolutions and contrast ratios of around 2000:1 are common. Other cinema standards lean toward different criteria, indicating that speckle “should not be visible”, this can be quantitatively assumed to have the level of speckle to be equivalent to that of a white light projector on a common screen.
Speckle noise can be quantified in terms of speckle contrast, C, given in percent as:
                    C        =                  100          ⁢                      (                                          I                std                                            I                mean                                      )                                              (        1        )            wherein Istd is a standard deviation of intensity fluctuation about a mean intensity Imean. The speckle contrast for fully developed speckle is 100%. Speckle reduces the ability of an imaging system to resolve fine spatial detail and causes levels of noise in an image that can be highly visually annoying. At worst, without some form of correction, speckle can be sufficiently objectionable to render coherent illumination unsuitable for display purposes.
There have been a number of methods employed for reducing the visibility of speckle effects in imaging displays. Conventional strategies for speckle reduction include modifying the spatial or temporal coherence of the illumination, superimposing a number of uncorrelated speckle patterns onto each other, or modifying its polarization state. One method provides vibration or oscillatory movement of the display screen. With oscillation above a threshold speed, perceived speckle can be significantly reduced. Other methods include broadening the spectral line width of the laser illumination and reducing the spatial coherence by using static and oscillating diffusers or oscillating fibers or by vibrating various optical components in the path of illumination or imaging light.
Goodman has characterized some common approaches to reducing speckle in display applications:                1. Introduce polarization diversity;        2. Introduce a moving screen;        3. Introduce a specially designed screen that minimizes the generation of speckle;        4. For each color, broaden the spectrum of the sources or use multiple lasers at slightly different frequencies, thereby achieving wavelength diversity in the illumination;        5. For each color, use multiple independent lasers separated spatially, thereby achieving angle diversity in the illumination;        6. Overdesign the projection optics as compared with the resolution of the eye;        7. Image a changing diffuser with random phase cells onto the screen; and        8. Image a changing diffuser with deterministic or orthogonal phase codes onto the screen.        
Each of these approaches has some benefits as well as negative attributes. Some of them apply well for high-end digital cinema projection, while others do not. In addition, in many cases a single approach may not be effective enough to reduce the speckle below acceptable thresholds. For example, polarization diversity is not desirable in many cases, as any projector that requires polarization either to modulate the light or to create stereoscopic imaging cannot allow impure states to reach the viewer. Specially designed screens that enable screen shaking can be effective, however, they require significant modification to the venue that is undesirable. Large screens are especially difficult to modify to enable screen shaking, as the equipment is large and expensive.
Spectrally broadening of the light sources can substantially reduce the level of speckle, however, this may be difficult to control in the laser fabrication, as many methods of creating visible solid state sources desired for display applications use frequency double crystals that control the wavelength to around 1 nm.
Multiple independent lasers can be a very good approach, but depends on the number of elements used to control the speckle. This does not work well over the entire range from low-light-level to high-light-level projection system, as a 1000 lumen projector needs to be as speckle free as a 10,000 lumen projector, yet the number of sources may be 10 times as high. For example, Mooradian et al, disclose improved speckle performance when using Novalux Extended Cavity Surface Emitting Lasers (NECSELS), in the article “High power extended vertical cavity surface emitting diode lasers and arrays and their applications” (Micro-Optics Conference, Tokyo, Japan, 2005). In this case 30 to 40 independent (incoherent to each other) emitters reduced the speckle down to several percent. While the speckle is reduced with larger number of emitters it is not always reduced to white light levels required by the stringent digital cinema requirements.
In U.S. Pat. No. 7,296,897, Mooradian et al., entitled “Projection display apparatus, system, and method,” discloses individual and combined techniques to reduce laser speckle similar to those described by Goodman. First increasing the number of lasers that are substantially incoherent with respect to each other. Second, spectral broadening of the lasers may be used. (This technique is also described in U.S. Pat. No. 6,975,294 to Manni et al.) Third, individual lasers in an array may be designed to operate with multiple frequencies, phase, and directional (angular) distributions. Finally an optical element may be used to scramble the direction, phase and polarization information. As described earlier, increasing the number of lasers is effective at reducing speckle, however the effect is incomplete. The additional methods described are generally difficult to implement, expensive or undesirable optically.
U.S. Pat. No. 7,244,028 to Govorkov et al., entitled “Laser illuminated projection displays,” describes the use at least one laser delivered to a scanning means that increases the laser beam divergence temporally into a lens that delivers the light to a beam homogenizer that illuminates a spatial light modulator. This reduces the laser speckle to acceptable levels when combined with a screen that has at least one feature to further reduce speckle. Temporally varying the laser beam divergence is generally a good means of reducing speckle, however it too requires the modification of the screen for complete speckle reduction. This is undesirable for general projection purposes.
U.S. Pat. No. 7,116,017 to Ji et al., entitled “Device for reducing deterioration of image quality in display using laser,” describes a specific device consisting of a vibrating mirror in the light path between the laser and the screen. This alone will not reduce speckle to acceptable levels. Commonly assigned U.S. Pat. No. 6,445,487 to Roddy et al., entitled “Speckle suppressed laser projection system using a multi-wavelength Doppler shifted beam,” describes methods that use frequency modulation of the lasers in conjunction with a device to deviate the beam angularly in time. This method requires laser modulation that may not be practical or possible for all laser sources. Similarly the application focuses on using an acousto-optic modulator for angular deviation. These devices are very expensive and can only handle certain laser types and sizes.
Numerous methods for reducing speckle have been described in the prior art. U.S. Pat. No. 6,747,781 to Trisnadi et al., entitled “Method, apparatus, and diffuser for reducing laser speckle,” discloses moving a diffusing element that is positioned at an intermediate image plane that subdivides image pixels into smaller cells having different temporal phase. Commonly-assigned U.S. Pat. No. 6,577,429 entitled “Laser projection display system” to Kurtz et al. discloses using an electronically controllable despeckling modulator to provide controllable, locally randomized phase changes with a linear SLM. U.S. Pat. No. 6,323,984 entitled “Method and apparatus for reducing laser speckle” to Trisnadi et al. discloses speckle reduction using a wavefront modulator in the image plane. U.S. Pat. No. 5,313,479 entitled “Speckle-free display system using coherent light” to Florence discloses illumination of a light valve through a rotating diffuser. U.S. Pat. No. 4,256,363 to Briones, entitled “Speckle suppression of holographic microscopy,” and U.S. Pat. No. 4,143,943 to Rawson, entitled “Rear projection screen system,” each disclose apparatus that reduce speckle by moving diffusive components that are within the projection path. Commonly-assigned U.S. Patent Application Publication 2009/0284713 to Silverstein, et al., entitled “Uniform speckle reduced laser projection using spatial and temporal mixing,” teaches using a temporally varying optical phase shifting device in the optical path to reduce speckle in a digital cinema system.
While conventional methods for speckle reduction may have some applicability to laser-based projection systems, there are drawbacks to these approaches that constrain image quality and reduce overall contrast as well as adding cost and complexity to projection apparatus. Any type of modification to components in the imaging path, for example, can require significant redesign, can complicate component packaging, and risks the introduction of noise or vibration into optical and signal paths of projector components.
The problem of speckle reduction is further complicated because different types of spatial light modulators (SLMs) are being used for digital projection. Three types of SLMs are used in practice: point-scan, line-scan and frame-by-frame. Point-scan projectors display an image by raster scanning a single pixel at a time. A number of projectors use grating light valves (GLVs) or grating electromechanical systems (GEMS) that generate images using diffractive gratings that have tiny mechanical members that are variably actuated in order to form an image. The image from such a device is scanned onto the display surface, a single line at a time. These modulators are advantaged with respect to simplicity and cost, and therefore are desirable for use in consumer devices such as pico-projectors. However, they present problems due to the energy density that can be delivered which limits the amount of light that can safely be projected. Other projectors employ reflective or transmissive liquid-crystal devices (LCDs). These SLMs project a complete image frame at a time. Still other projection apparatus use digital micromirror devices with two-dimensional arrays of micro-electromechanical reflectors, such as the Digital Light Processor (DLP) from Texas Instruments, Inc., Dallas, Tex. DLP devices similarly form a complete image frame at a time. These area-type devices are advantaged in delivering less energy density to the screen offering safer operation. Because images are formed in different ways using these different SLMs and projection technologies, solutions that compensate for speckle with one type of SLM may not be as effective when used in a projector that uses a different type of SLM for forming images.
A number of different approaches have been developed which use specially designed screens to reduce speckle. U.S. Pat. No. 6,122,023 to Chen et al., entitled “Non-speckle liquid crystal projection display,” discloses a projection screen which includes a liquid crystalline material. When driven with an AC voltage the liquid crystalline materials vibrate slightly which causes the speckle pattern to change quickly which causes the observed speckle noise by the viewer to be reduced.
U.S. Pat. No. 7,304,795 to Yavid and Stern, entitled “Image Projection with Reduced Speckle Noise,” discloses a projection screen which includes a plurality of optical resonator cavities which trap incident laser light for a time greater than the coherence time and for generating a time varying interference pattern in which speckle noise is reduced.
U.S. Pat. No. 5,473,469 to Magocs and Baker, entitled “Front projection screen with lenticular front surface,” discloses a front projection screen for use with a laser projector which includes a lenticular lens array on the front surface of the screen which incorporates light scattering particles to form a diffusion region and a reflector on its back surface. Since incident light rays traverse different portions of the diffusion region in different directions which increases the likelihood that the ray will incorporate a scattering particle, speckle noise is reduced.
The use of projection screens incorporating color changing materials is described in the following art. U.S. Pat. No. 7,414,621 to Yavid et al., entitled “Method and Apparatus for Controllably Producing a Laser Display,” discloses a raster scanned laser display for projecting an image on a screen incorporating at least one phosphor at the screen for reflecting light with a wavelength different from the wavelength of the incident laser beam which emits light in the ultra-violet or IR wavelength region of the spectrum. Complete absorption of the laser beam is required by the phosphor in order to fully utilize this approach.
U.S. Pat. No. 6,987,610 to Piehl, entitled “Projection Screen,” discloses a projection screen comprising a substrate having thereon one or more fluorescent materials that emit visible light with an incidence of one or more ranges of visible light and absorb visible light in at least one other range of wavelengths that is not included in the one or more ranges and one or more absorption materials disposed between the substrate and the one or more fluorescent materials that reflect wavelengths of light in the one or more ranges and absorb wavelengths of light that are not included in the at least one other range nor in the one or more ranges.
U.S. Patent Application Publication 2008/0172197 to Skipor et al., entitled “Single laser multi-color projection display with quantum dot screen,” discloses a display comprising a passive screen printed with a pattern of different color quantum dots that is excited by raster scanning a single UV laser beam over the screen.
U.S. Pat. No. 7,474,286 to Hajjar et al., entitled “Laser Displays using UV-Excitable Phosphors Emitting Visible Colored Light,” discloses a display system using at least one scanning laser beam to excite one or more fluorescent materials on a screen in the form of parallel phosphor stripes which emit light to form images. An alignment verification sensor is also required to verify that the laser light modulation timing is correctly aligned with the phosphor stripes during raster scanning of the laser over the screen surface. In a related disclosure, U.S. Patent Application Publication 2008/0291140 to Kent et al., entitled “Display Systems Having Screens with Optical Fluorescent Materials,” further teaches that the fluorescent materials may include phosphor materials or quantum dots.
U.S. Patent Application Publication 2008/0048936 to Powell et al., entitled “Display and display screen configured for wavelength conversion,” discloses a display screen including an array of couplets containing a wavelength converting material. The couplets are configured to receive light at a first wavelength and responsively emit light at a second wavelength preferentially in a direction.
U.S. Patent Application Publication 2009/0262308 to Ogawa, entitled “Light source unit and projector,” discloses a projector, which includes first and second light sources comprising light emitting diodes or a solid-state light emitting devices for emitting light in each of two predetermined wavelength bands and a third light source formed by a phosphor which transmits light of the first light source and absorbs light emitted from the second light source. In this case there is no phosphor material on the screen.
Thus, it can be appreciated that speckle presents a recurring problem that must be addressed in projection apparatus design when laser illumination is used. Conventional speckle compensation approaches add cost and complexity to projector design, and generally reduce image quality with respect to projector output. There is, then, a need for a speckle compensation mechanism that can be used for a broad range of imaging technologies and that does not impact projector design.